Field of the Invention:
The present invention relates to a transparent photorechargeable electrochemical device, to the use of an n-type semiconductor as positive-electrode active material in said transparent photorechargeable electrochemical device, and to a method for photorecharging said device. In particular, it relates to the field of alkaline batteries, and especially to lithium batteries.
Description of Related Art:
Lithium batteries have become essential components in many devices including portable appliances such as, in particular, mobile phones, computers and handheld tools, or heavier machinery such as two-wheeled means of transportation (bicycles, mopeds) or four-wheeled means of transportation (hybrid or electric automotive vehicles). They have also been widely studied for use in the field of power storage.
A lithium battery comprises at least one negative electrode and at least one positive electrode between which is placed a solid electrolyte or a separator impregnated with a liquid electrolyte. The liquid electrolyte for example consists of a lithium salt in solution in a solvent chosen to optimize the transport and disassociation of ions. The positive electrode consists of a current collector bearing an electrode material that contains at least one positive-electrode active material capable of Inserting lithium ions reversibly; the negative electrode consists of a sheet of lithium metal (optionally carried by a current collector), of a lithium alloy or of an intermetallic lithium compound (lithium battery), or of a current collector carrying an electrode material that contains at least one negative-electrode active material capable of inserting lithium ions reversibly (lithium-ion battery)
During operation of the battery, lithium ions pass from one of the electrodes to the other through the electrolyte. During the discharge of the battery, a quantity of lithium from the electrolyte reacts with the positive-electrode active material, and an equivalent quantity is introduced into the electrolyte from the active material of the negative electrode, the lithium concentration thus remaining constant in the electrolyte. The insertion of lithium into the positive electrode is compensated for by electrons from the negative electrode, supplied via an exterior circuit. During charging, the reverse effects take place.
The per-unit-weight energy densities currently obtained in various electrochemical energy-storage systems are 200-250 Wh/kg for the best Li-on batteries, 100-150 Wh/kg for an Na-ion battery, 350 Wh/kg for a Li—S battery, 500 Wh/kg for a lithium-air battery and 50 Wh/kg for a redox-flow battery. These batteries therefore have relatively low per-unit-weight energy densities (meaning the amount of deliverable power is limited) and must be recharged via the mains grid.
Moreover, over the last decade, devices allowing solar energy to be converted into electrical power have been proposed. In particular, Grätzel has developed a dye-sensitized solar cell (DSSc, DSC or DYSC) comprising a transparent, electrically conductive layer of fluorine-doped tin oxide SnO2.F, on which layer is placed a photoanode of titanium oxide TiO2, on the surface of which a photosensitive pigment (e.g. a ruthenium (+II) polypyridyl complex) is grafted; a platinum counterelectrode; and an iodide/triiodide electrolyte (I−/I3−). However, such a photovoltaic cell does not allow energy to be stored.
Other systems such as photocapacitors combining two technologies, i.e. a photovoltaic cell and a capacitor, have been developed in order to allow electricity directly converted from light energy to be stored. Generally, such systems comprise a photoelectrode; a bifunctional internal electrode, i.e. an electrode that functions both as a cathode and as an anode, and that is located between and in contact with two different electrolytes; and a counterelectrode in the capacitor portion. However, in order to maintain a good performance level, migration and/or diffusion of oxidoreductive species into the capacitor portion must be inhibited, creating difficulties in the manufacture of such systems and in the design of high-performance materials. Moreover, such systems have a high production cost. By way of example, US2009/0078307 describes a device combining two technologies: a DSSc photovoltaic cell and an anionic battery. However, the storage capacities achieved are relatively low since they are about 0.01 mAh/cm2.
Tributsch [J. Appl. Phys., 1987, 62, 11, 4597-4605] has therefore proposed photorechargeable batteries using semiconductors capable of reversibly photoinducing the intercalation of metal ions. In particular, Tributsch proposes a photorechargeable battery comprising a photoelectrode comprising a p-type semiconductor such as copper thiosulphate Cu3PS4, a counterelectrode such as a copper wire, and an electrolyte comprising a solution of copper chloride (CuCl) and of tetrabutylammonium perchlorate (TBAPC) in acetonitrile. However, the performance level obtained is limited, especially by the slow rate of the discharge reaction. Moreover, the use of copper does not allow high voltages to be achieved. Lastly, sulphur-based semiconductors are to be avoided since they generally induce, during the excitation by light waves, photoelectrolysis of water even if the electrolyte only contains a small amount thereof. Furthermore, such semiconductors may be corroded in the electrolyte.